1. Field of the invention
The present invention relates to the construction of an acceleration sensor particularly for use in an air bag system for an automobile.
2. Description of the Prior Art
One example of a conventional acceleration sensor is illustrated in FIG. 1. A conventional acceleration sensor 50 comprises an acceleration detecting element 51, a circuit board 52, a plurality of connecting terminals 53, a metal base plate 54, a metal cap 55, and insulating projections 56. The acceleration detecting element 51 is an element for detecting acceleration, converting the acceleration into an electric signal and outputting the electric signal and is carried on the circuit board 52. A signal processing circuit (not shown) for processing an acceleration signal detected by the acceleration detecting element 51 is also carried on the circuit board 52. An output of the signal processing circuit carried on the circuit board 52 is outputted outward from the acceleration sensor 50 through the plurality of connecting terminals 53. The circuit board 52 is bonded and fixed to the surface of the metal base plate 54. Each of the plurality of connecting terminals 53 has its end fixed to the circuit board 52 and the other end projected from the reverse surface of the metal base plate 54 after penetrating through the metal base plate 54. The metal cap 55 is fixed to a peripheral edge of the metal base plate 54 to cover the entire acceleration sensor 50. The connecting terminal 53 is insulated from the metal base plate 54 and is fixed to the metal base plate 54 in a portion, through which the connecting terminal 53 penetrates, of the metal base plate 54.
The acceleration sensor 50 is mounted on the surface of a mounting substrate 60 by inserting the plurality of connecting terminals 53 through mounting holes (not shown) of the mounting substrate 60 and connecting and fixing the connecting terminals 53 and a wiring pattern of the mounting substrate 60 to each other by soldering.
The conventional acceleration sensor 50 is used for detecting the change in acceleration in the travel direction of an automobile when it is used in an air bag system for an automobile, for example. However, vibration from a road surface and various types of vibration from the engine and the like are transmitted to the acceleration sensor 50 when it is carried on the body of the automobile. Such vibration may, in some cases, cause harmful vibration in the mounting substrate 60 and in the acceleration sensor 50. For example, if vibration in a bending mode is produced in the mounting substrate 60 by some external force, the mounting substrate 60 will be mechanically strained. If the mounting substrate 60 is strained, the effect of the strain is transmitted to the metal base plate 54 through the connecting terminals 53, whereby the metal base plate 54 is deformed. The deformation of the metal base plate 54 is further transmitted to the acceleration detecting element 51 through the circuit board 52 bonded to the surface of the metal base plate 54, to affect the acceleration detecting element 51. As a result, a malfunction occurs in the acceleration detecting element 51, whereby output sensitivity in a direction other than the direction in which acceleration is to be detected, for example, sensitivity in the other axial direction is produced, causing a malfunction in the air bag system.
Thus, a malfunction may occur due to the harmful vibration of the mounting substrate 60 in the conventional acceleration sensor 50 shown in FIG. 1.